1. Field of the Invention
The invention relates to a reflective projection display system and, more particularly, to a projection display system using reflective micro-mirrors.
2. Description of the Related Art
Recently, a technology of using reflective micro-mirrors to project images on large-scale monitors such as back projection televisions draws more and more attention and develops actively. The technology originated in the digital light processing technology, the imaging principle of which will be described in the following, developed by the Texas Instruments in 1987.
As shown in FIG. 1, the uniformly incident light rays 111 outputted from an illumination device 11 to enter a prism set 12 composed of two prisms 121 and 122 are reflected by the prism set 12 and then projected onto a DMD (Digital Micro-mirror Device) 13 composed of ten thousands of micro-mirrors (not shown). Each micro-mirror on the DMD 13 has its individual driving electrode and steering shaft. In a light-state mode, these micro-mirrors reflect the incident light rays 111a to let the incident light rays 111a pass the prisms 121 and 122 and enter a projection lens 141 that focuses the light rays onto a screen (not shown). On the other hand, in a dark-state mode, these micro-mirrors are tilted with a specific angle by applying voltages to their driving electrodes in order to lead the incident light rays 111a away from optical axis 15 of the projection lens 141 after the incident light rays 111a passes through the prism set 12.
However, since the deviation light rays 111c in the dark-state mode are output over the prism set 12, it is necessary to keep a predetermined distance LD between the prism set 12 and the projection lens 141 in order to prevent the projection lens 141 from receiving the deviation light rays 111c and to improve the contrast of the projection display system 1. Consequently, such a method can increase the back focal distance LB and thus enlarge the overall thickness LT of the projection display system 1; therefore, there are difficulties in a wide-angle design, which preferably needs shorter back focal distance, of the projection lens 141.
On the other hand, although a projection lens 141 with a small aperture φp can be used to lower the design and manufacturing costs of the projection lens 141 and thus to shorten the distance LD between the projection lens 141 and the prism set 12 and to decrease the overall thickness LT of the projection display system, the reduction of the distance LD between the projection lens 141 and the prism set 12 is always limited by the fact that the deviation light rays 111c are always output from the top of the prism set 12. Therefore, it is difficult to effectively reduce the overall thickness of the projection display system 1.
Consequently, it is necessary to find a low-cost solution for enhancing the contrast of the projection display system as well as reducing the overall thickness of the projection display system in these days when the image quality requirements for the large-scale monitor of the projection display system is getting more and more demanding in the consumer market.
The invention discloses a projection display system using reflective micro-mirrors and shortening the back focal distance of the projection display system and reducing the overall thickness of the projection display system without damaging high brightness and high contrast in the projection display system to fundamentally solve the problems of the prior art.